Tons of weight? Whats a battery weigh? 2 or 3oz?

As much as I hate to add lead, I'd put lead on the engine box before I'd put my RX battery right next to the ignition battery.

It's always a bad idea to put your ignition and RX components within 12" of each other. The batteries, switches, RX, ignition--etc.. should all be 12" away from each other.

If you want to risk it, then do it. Just do a good range check before you fly. Use a standard FM PPM RX to range check it. The PPM RX won't hide any problems that you might encounter with your setup. Once you get a good range check--then switch to the PCM RX if you want too. But don't use a PCM RX to do your initial range checks. It will hide any problems you might have. Then those problems will sneak up and bite you later when the RX locks up.

Good luck.